This invention provides an improved process for separating a hydrocarbon bearing gas stream containing significant quantities of components more volatile than methane (e.g., hydrogen, nitrogen, etc.) into two fractions: one fraction contains predominantly methane and the more volatile components and the second fraction contains the recovered desirable ethylene/ethane and heavier hydrocarbons components.
Ethylene/ethane, propylene/propane and heavier hydrocarbons can be recovered from a variety of gases, such as natural gas, refinery gas and synthetic gas streams obtained from other hydrocarbon materials such as coal, crude oil, naphtha, oil shale, tar sands and lignite. Hydrocarbon bearing gas typically contains components more volatile than methane (e.g., hydrogen, nitrogen, etc.) and often unsaturated hydrocarbons (e.g., ethylene, propylene, etc.) in addition to methane, ethane and hydrocarbons of higher molecular weight such as propane, butane and pentane. Sulfur-containing gases and carbon dioxide are also sometimes present. The present invention is generally concerned with the recovery of ethylene/ethane and heavier (C.sub.2 +) hydrocarbons from such gas streams.
Recent changes in ethylene demand have created increased markets for ethylene and derivative products. In addition, fluctuations in the prices of both natural gas and its NGL constituents have increased the incremental value of ethane and heavier components as liquid products. These market conditions have resulted in a demand for processes which can provide high ethylene recovery and more efficient recoveries of all these products. Available processes for separating these materials include those based upon cooling and refrigeration of gas, oil absorption and refrigerated oil absorption. Additionally, cryogenic processes have become popular because of the availability of economical equipment which produces power while simultaneously expanding and extracting heat from the gas being processed. Depending upon the pressure of the gas source, the richness (C.sub.2 + content) of the gas and the desired end products, each of these processes or a combination thereof may be employed.
The cryogenic expansion process is now generally preferred for ethylene/ethane recovery because it provides maximum simplicity with ease of start up, operating flexibility, good efficiency, safety and good reliability. U.S. Pat. Nos. 4,061,481, 4,157,904, 4,171,964, 4,27S,457 and 4,617,039 describe relevant processes.
In a typical cryogenic expansion recovery process, a feed gas stream under pressure is cooled by heat exchange with other streams of the process and/or external sources of refrigeration such as a propane compression-refrigeration system. As the gas is cooled, liquids may be condensed and collected in one or more separators as high-pressure liquids containing some of the desired C.sub.2 + components. Depending on the richness of the gas and the amount of liquid formed, the high-pressure liquids may be expanded to a lower pressure and fractionated. The vaporization occurring during expansion of the liquid results in further cooling of the stream. Under some conditions, pre-cooling the high pressure liquid prior to the expansion may be desirable in order to further lower the temperature resulting from the expansion. The expanded stream, comprising a mixture of liquid and vapor, is fractionated in a distillation (demethanizer) column.
If the feed gas is not totally condensed (typically it is not), the vapor remaining from the partial condensation is passed through a work expansion machine or engine, or an expansion valve, to a lower pressure at which additional liquids are condensed as a result of further cooling of the stream. The pressure after expansion is usually the same as the pressure at which the distillation column is operated. The combined vapor-liquid phases resulting from the expansion are supplied as feed to the column.
In the column, the expansion-cooled stream(s) is (are) distilled to separate residual methane, nitrogen, and other volatile gases as overhead vapor from the desired ethylene/ethane, propylene/propane and heavier components as bottom liquid product.
In the ideal operation of such a separation process, the residue gas leaving the process will contain substantially all of the methane and more volatile components in the feed gas with essentially none of the heavier hydrocarbon components and the bottoms fraction leaving the demethanizer will contain substantially all of the heavier components with essentially no methane or more volatile components. In practice, however, this ideal situation is not obtained for the reason that the conventional demethanizer is operated largely as a stripping column. The residue gas from the process, therefore, typically comprises vapors leaving the top fractionation stage of the column, together with vapors not subjected to any rectification step. Considerable losses of ethylene/ethane (C.sub.2) occur because the top liquid feed contains substantial quantities of C.sub.2 + components, resulting in
corresponding equilibrium quantities of C.sub.2 + components in the vapors leaving the top fractionation stage of the demethanizer. This problem is exacerbated if the gas stream(s) being processed contain relatively large quantities of components more volatile than methane (e.g., nitrogen, hydrogen, etc.), because the volatile vapors rising up the column strip C.sub.2 + components from the liquids flowing downward. The loss of the desirable C.sub.2 + components could be significantly reduced if the rising vapors could be brought into contact with a significant quantity of liquid (reflux), containing very little C.sub.2 + components; that is, reflux capable of absorbing the C.sub.2 + components from the vapors. The present invention provides the means for achieving this objective and significantly improving the recovery of the desired products.
In accordance with the present invention, it has been found that ethylene recoveries in excess of 99 percent can be obtained. In addition, the present invention makes possible higher ethylene and ethane recoveries (up to several percentage points higher) than prior art processes with the same energy requirements. The present invention is particularly advantageous when processing feed gas that contains more than 10 mole % of components more volatile than methane (e.g., hydrogen, nitrogen, etc.).